A direct-sequence or direct sequence coding spread spectrum communication technique in essence combines two digital signals, or bit streams, to create a third signal prior to transmission. The first signal is an information signal, such as the output of a digitized voice circuit. For example, the first signal may have a bit rate of 10 kb/s. The second signal is generated by a random-sequence, or pseudonoise (PN) generator, and is a stream of essentially random bits having a bit rate that is several orders of magnitude greater than the bit rate of the digitized voice signal. The modulation of these two signals results in the third signal having the same bit rate as the second signal. However, the third signal also contains the digitized voice signal. At the receiver, an identical random-sequence generator produces a random bit stream which mirrors the original random-sequence that was used for modulation at the transmitter. For proper operation, after carrier frequency de-modulation, the PN generator of the receiver must be synchronized to the incoming PN sequence. By removing the random sequence from the received signal and integrating it over a symbol period, a despread signal is obtained. Ideally, the despread signal exactly represents the original 10 kb/s voice signal.
The TIA/EIA Interim Standard, Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, TIA/EIA/IS-95 (July 1993) specifies in Section 6.1.2 that a mobile station shall provide two independent techniques for output power adjustment. These two techniques are an open loop estimation based solely on mobile station operation, and a closed loop correction that involves both the mobile station and the cell site controller, or base station. In the latter technique the mobile station responds to power control bits received over a Forward Traffic Channel to adjust its output power level. In the former technique the received signal strength from the base station is used.
The power control in the CDMA system is also described at pages 10 and 12, and shown generally in FIG. 3-2, in a publication entitled "Introduction to CDMA and the Proposed Common Air Interface Specification (CAI) for a Spread Spectrum Digital Cellular Standard-An Overview of the Application of Code Division Multiple Access (CDMA) to Digital Cellular Systems and Personal Cellular Networks", QUALCOMM Incorporated, Mar. 28, 1992. As is described in this publication, the goal of the mobile station transmitter power control process is to produce, at a cell site receiver, a nominal received signal power from each mobile station transmitter that is operating within the cell. If all mobile stations are so controlled, the end result is that the total signal power received at the cell site from all of the mobile stations is equal to the nominal received power times the number of mobile stations.
It can therefore be appreciated that the control of the transmitter power is an important consideration when designing the mobile station, such as a radiotelephone, for operation in the CDMA telecommunications system.
Furthermore, in that the open loop power control relies on the signal received from the cell site by the mobile station, the operation of the mobile station receiver plays an important role in the correct operation of the open loop power control. In particular, the operation of the receiver Automatic Gain Control (AGC) function must be carefully considered.
In the CDMA system the receiver is required to operate over an 80 dB range. However, and because of the high sampling rate, the number of bits of resolution of a receiver analog to digital (A/D) converter is limited. Further complicating the problem caused by the limited resolution of the A/D converter, the receiver AGC function must also accommodate received signal fluctuations due to both slow and fast fades.
In the CDMA system the fast AGC function should not impair the functioning of the receiver algorithms and, ideally, should not impair the information collected for convolutional decoding and synchronization acquisition.
The CDMA specification also specifies the behavior of the mobile station transmitter. The reaction time of the transmitter power to a change in the received signal level is specified to be 30 ms, after which time the transmitter power level should be settled within new limits. Limits are also specified for the transition period. However, the specified transmitter response time constant of 30 ms is too lengthy for the fast receiver AGC function, and thus precludes a solution where both the transmitter and the receiver AGC settings would be equal.
In addition, the accuracy of the transmitter gain setting is tightly specified by the CDMA specification. To fulfill the specification, a transmitter power step size of 0.25 dB is required in the transmitter. In contrast, the receiver is rather tolerant to inaccuracy in the gain setting, thus making a less complicated and less costly solution possible. In addition, the receiver requires a larger step size than 0.25 dB to make a higher tracking rate possible.
The following U.S. Patents and other publications generally pertain to the teaching of this invention.
U.S. Pat. No. 5,168,505 to Akazawa et al., issued Dec. 1, 1992 and entitled "AUTOMATIC GAIN CONTROL DEVICE FOR SPREAD SPECTRUM COMMUNICATION DEVICE".
U.S. Pat. No. 5,107,225 to Wheatley, III et al., issued Apr. 21, 1992 and entitled "HIGH DYNAMIC RANGE CLOSED LOOP AUTOMATIC GAIN CONTROL CIRCUIT".
U.S. Pat. No. 5,093,840 to Schilling, issued Mar. 3, 1992 and entitled "ADAPTIVE POWER CONTROL FOR A SPREAD SPECTRUM TRANSMITTER".
U.S. Pat. No. 5,099,204 to Wheatley, III, issued Mar. 24, 1992 and entitled "LINEAR GAIN CONTROL AMPLIFIER".
U.S. Pat. No. 5,132,985 to Hashimoto et al., issued Jul. 21, 1992 and entitled "SPREAD SPECTRUM RECEIVER".
U.S. Pat. No. 5,056,109 to Gilhousen et al., issued Oct. 8, 1991 and entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM".
U.S. Pat. No. 5,265,119 to Gilhousen et al., issued May 17, 1991, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM".
U.S. Pat. No. 4,993,044 to Akazawa, issued Feb. 12, 1991 and entitled "SPREAD-SPECTRUM COMMUNICATION RECEIVER".
U.S. Pat. No. 4,901,307 to Gilhousen et al., issued Feb. 13, 1990 and entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS".
PCT International Application No. WO 93/1060, published 27 May 1993 and entitled "ADAPTIVE POWER CONTROL FOR A SPREAD SPECTRUM COMMUNICATIONS SYSTEM AND METHOD".
PCT International Application No. WO 93/07702, published 15 Apr. 1993 and entitled "TRANSMITTER POWER CONTROL SYSTEM".
PCT International Application No. WO 93/05585, published 18 Mar. 1993 and entitled "A METHOD FOR AUTOMATIC TRANSMISSION POWER CONTROL IN A TRANSCEIVER SUITABLE FOR A CDMA ENVIRONMENT EMPLOYING DIRECT SEQUENCE DIFFUSION".